Rollup merge of #139465 - EnzymeAD:autodiff-sret, r=oli-obk

add sret handling for scalar autodiff

r? `@oli-obk`

Fixing one of the todo's which I left in my previous batching PR.
This one handles sret for scalar autodiff.  `sret` mostly shows up when we try to return a lot of scalar floats.
People often start testing autodiff which toy functions which just use a few scalars as inputs and outputs, and those were the most likely to be affected by this issue. So this fix should make learning/teaching hopefully a bit easier.

Tracking:

- rust-lang/rust#124509

Author: Zalathar